Two cosmic anomalies tell us that something big is missing from our model of the Universe. First, stars in the outer regions of a typical galaxy are orbiting the centre too fast for the galaxy’s gravity to hold onto them. By rights, they should fly off into intergalactic space.
The second anomaly is that you are reading these words – that is, galaxies like the Milky Way, and therefore you, exist. According to the standard picture of galaxy formation, regions of the cooling debris of the Big Bang that were slightly denser than average would have had slightly stronger gravity and pulled in material faster, enhancing their gravity so they pulled in matter even faster, and so on.
But this process – akin to the rich getting ever richer – could not have built galaxies as big as our Milky Way in the 13.8 billion years that the Universe has existed.
Confronted with these anomalies, most astronomers postulated that the Universe contains about five times as much invisible matter as visible stars and galaxies. It is the extra gravity of such ‘dark matter’, they claim, that holds onto stars in galaxies and sped up galaxy formation. However, an equally logical possibility is that, on cosmic scales, gravity is stronger than Newton would have predicted.
In 1981, the Israeli physicist Prof Mordechai Milgrom found that the anomalously orbital motion of stars in the outer regions of galaxies could be explained if they were experiencing a stronger form of gravity.
This would mean that gravity weakens less quickly with distance than the Newtonian theory of gravity predicts, and ‘switches’ to this form when the stars are experiencing a particular threshold acceleration towards the centre of their galaxies. Thus was born the hypothesis known today as modified Newtonian dynamics, or MOND.
Stars are always experiencing an acceleration towards the centre of a galaxy. This is called centripetal acceleration. Gravity must provide this acceleration to keep them in orbit. The point is that in MOND, gravity switches to the stronger form at a threshold acceleration of 10-10 m/s2, which is generally found in the outer regions of big galaxies.
The majority of astronomers, however, persisted with the dark matter idea, and it has become an integral part of the standard model of cosmology, known as Lambda-CDM. Lambda refers to the mysterious ‘dark energy’ that is speeding up the expansion of the Universe, and CDM to ‘cold’ dark matter. Because CDM consists of particles moving sluggishly, it is gathered into clumps by gravity – clumps which then pull in ordinary matter to make visible galaxies.
Now, physicists led by Dr Indranil Banik at St Andrews University in Scotland are claiming that observations of the Universe can, in fact, be better explained by a modification of our current theory of gravity than by dark matter.
Lambda-CDM is very good at explaining what we observe, they say. “But this is usually after the event,” says Banik. “MOND has been better at predicting things in advance of observations.”
One apparent shortcoming of MOND is that it still needs an element of dark matter to explain the motions of galaxies in galaxy clusters – possibly a hypothetical heavy particle known as a sterile neutrino. However, Banik does not see this as necessarily a problem.
“In our Solar System, the anomalous orbits of two planets required new explanations,” he says. “For Uranus, it was the pull of a new planet, Neptune – the original dark matter. For Mercury, it was a new theory of gravity, namely Einstein’s.”
The main thesis of Banik and his St Andrews colleague is that there are several observations that dark matter cannot explain, but that modified gravity can. For instance, the former predicts that satellite galaxies should be distributed spherically, like a swarm of bees – but in many galaxies, including our own, they orbit in a single plane.
Also, the bar-shaped structures made of stars that are seen in the heart of some spiral galaxies should be slowed by a ‘dark matter bar’ rotating just behind them. “However, in 42 bars whose speeds have been measured, this has not been seen,” says Banik.
Proponents of dark matter, on the other hand, see these things as discrepancies that will eventually be explained, not as fatal flaws in the paradigm.
“A lot of interlinked observations make sense only with dark matter,” says Prof James Peebles of Princeton University, who won the Nobel Prize for the cold dark matter theory.
“That is not to say that the Lambda-CDM theory is the whole truth; but it is a good approximation.”
Banik disagrees. However, he does not think experimenters looking for dark matter particles on Earth should give up; merely that they should design future experiments so that, even if they fail to find dark matter candidates, they reveal something important about nature.
“For instance, a search for sterile neutrinos, even if they are not found, will tell us about neutrinos,” Banik says.
“Since their properties are not predicted by the Standard Model of particle physics, anything we discover would give us hints at the deeper Theory of Everything, of which the Standard Model is thought to be an approximation.”
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